The present invention relates generally to a coordinate measuring machine (CMM) for determining a measuring position of a probe and a method for determining a measuring position of a probe of a coordinate measuring machine (CMM).
There is a demand for high precision position measurement of object surface points in many technical fields. In particular for the manufacturing industry the measurement and quality control of produced objects have high significance.
In connection with these fields there exist a number of measuring devices which are formed for such special tasks, usually being called coordinate measuring machines (CMM). One type of such devices is an articulated arm CMM, with some embodiments of such articulated arm CMMs being exemplarily described in U.S. Pat. Nos. 5,402,582, 7,051,450 and EP 1 474 650. Comparable systems are offered, for example, by the firm “ROMER” as “SIGMA”, “FLEX” or “OMEGA” and by the firm “CIMCORE” as “INFINITE” or “STINGER”.
A conventional three-dimensional articulated arm CMM is composed of a manually operated multi-jointed articulated arm having a support base on one end and a measurement probe at the other end thereof. A host computer may communicate with the arm via an intermediate controller or serial box.
Articulated arm CMMs are usually formed in such a way that the measurement probe is freely movable in a specified volume. Thereby, a counterbalance system may be provided so that a user can move the arm easily and with small forces. One possibility of providing a counterbalance is to install counter-weights on the arm components. Another possibility is to use spring forces or gas cylinders to provide the counterbalance.
Each joint of the articulated arm is assigned at least one sensing unit for measurement of an actual relative setting of the respective joint. Therefore, for example, optoelectronic angle sensors are used.
By combining the measured relative settings of each joint of the arm the exact three-dimensional position of a surface point of interest, e.g. which is touched by the measurement probe, can be derived relative to the base. Thus, the coordinates of this surface point can be calculated in a machine-internal coordinate system.
In many applications, however, the exact position of the surface point is needed relative to an external coordinate system, for example in a defined measurement room or working area. Therefore, a couple of methods are known for referencing or matching the determined machine-internal coordinates of the measuring points relative to the external coordinate system, in particular in connection with portable or moveable articulated arm CMMs.
For example, once the external position of the base is known—assuming that the base of the arm is placed horizontally—the measured internal positions can be transferred into the external coordinate system and, thus, referenced in the measurement room or working area.
In this context, the internal position of a point shall be understood as the position of said point given in the machine-internal coordinate system and, therefore, relative to the base of the CMM. Accordingly, the external position of a point means the position of the point in the external coordinate system, thus making it possible to specify a position of interest in a defined external measurement room or working area.
In order to provide the possibility of referencing internal positions relative to an external measurement room, reference points may be installed on the floor of the measurement room, the exact positions of the reference points being known in the external coordinate system. Preferably, the reference points are positioned in a net-forming manner so that at least three defined reference points can be reached by the probe of the articulated arm CMM wherever the base of the CMM is positioned in the measurement room. Hence, by measuring at least three reference points by the articulated arm CMM, the external position of the base can be recalculated and, thereafter, any measured internal position of surface points of interest can be referenced in the external coordinate system.
The base of the CMM is built for being positionable on a supporting surface, for example the floor of a measuring room or a table. Therefore, the base usually is built very stably and rigid. Exemplarily, the base can comprise a planar bottom side in order to put the CMM rigidly on the supporting surface. Alternatively, stands, in particular trolley stands, are used as part of the base for placing the articulated arm CMM on a desired position on the floor.
Thereby, the base can be understood as that part of the articulated arm CMM which is not moved while operating the articulated arm during a measurement, e.g. while directing the probe of the arm towards the point to be measured.
According to articulated arm CMMs of the state of the art, it is assumed that the base has a completely rigid stand on the supporting surface and that the position and the slope of the base don't vary during measurements. Therefore, using a determined or given external position of one known point of the base would be feasible for referencing measured internal positions of points of interest relative to the external coordinate system.
However, it has turned out that varying load conditions of the supporting surface—whereupon the base is placed—influence the precision and accuracy of the determined external positions of measured points of interest.
Furthermore, a torque or a force which acts not perpendicular to the supporting surface onto the base of the articulated arm during measurements may introduce an error in case that the support and/or the base are not rigid enough. This may happen if the counterbalance system of the arm uses spring forces for providing a balanced arm, as such a counterbalance system may introduce varying and not straight down acting forces onto the base while operating and moving the probe of the arm.
It is therefore an object of the present invention to provide an improved articulated arm CMM, in particular wherein the above mentioned disadvantages of machines of the art are reduced or eliminated. A further object is to allow a more precise and less troublesome referencing of a measured point of interest relative to an external coordinate system, particularly in case of varying load conditions of the supporting surface whereupon the base is placed.
Those objects are achieved by realising the features of the independent claims. Features which further develop the invention in an alternative or advantageous manner are described in the dependent patent claims.